Micro-contact printing method

ABSTRACT

The invention relates to micro-contact printing, wherein a self-assembled monolayer(SAM)-forming molecular species ( 1 ) is applied to a surface ( 2 ) of an article ( 3 ). The SAM-forming species ( 1 ) comprise a polar functional group that is exposed when the species ( 1 ) form a monolayer. This enables said printing method to be performed in vacuum or in a gaseous atnosphere, preferably in air. The invention also relates to an article having a surface comprising at least one isolated region of a SAM having a lateral dimension within the range of from 1 to 100 nm. Furthermore, the invention relates to a method for producing at least one nanowire, or a grid of nanowires, having a lateral dimension within the range of from 1 to 100 nm.

The present invention relates to a method of applying a self-assembledmonolayer of a molecular species to a surface of an article.

The invention also relates to an article having a surface comprising atleast one isolated region of a self-assembled monolayer of a molecularspecies.

Furthermore, the invention relates to a method for producing at leastone nanowire, or a grid of nanowires.

In the manufacturing of microelectronic and optical devices, thetransferring of a pattern in the micro and/or nano scale regions to asurface of an article made of a conducting, insulating orsemi-conducting material is a crucial process. Such a process should becontrollable, and conveniently and inexpensively reproducible with arelatively low failure rate.

A well-known technique for transferring a pattern to an article isphotolithography. A negative or positive photoresist is first coatedonto the surface of the article. The resist is then irradiated inaccordance with a predetermined pattern and irradiated (positive resist)or non-irradiated (negative resist) resist portions are washed away fromthe surface to give a predetermined pattern of resist on the surface.The resist may then serve as a mask in an etching process wherein thesurface of the article that is not covered by the resist is etched, andafter removal of the resist, a predetermined pattern of non-etchedconducting, insulating or semi-conducting material is obtained on thesurface of the article.

However, photolithography requires relatively advanced and expensiveapparatus and is also relatively time-consuming.

Another method for transferring a pattern to an article is micro-contactprinting. There are two main printing principles known in the art, andseveral variants thereof, involving micro-contact printing.

The first printing principle, herein referred to as “standard printing”,comprises pressing two sheets against each other, said two sheetscontacting each other through a plane. In a variant of this printingprinciple use is made of a stamp for transferring a pattern from astamping surface (first “sheet”) to an article surface (second “sheet”).A modification of the stamping process is, for instance, a printingmethod wherein a slightly curved stamping surface is used. Anotherexample is a printing method wherein parts of a flexible stampingsurface contact the article surface sequentially.

The second printing principle, herein referred to as “roll printing”,comprises rolling of a cylinder along a sheet, wherein the cylinder andthe sheet contact each other along a line.

However, other micro-contact printing principles, or variants andmodifications thereof, are also possible.

WO 96/29629 describes a printing process wherein a self-assembledmolecular monolayer is formed on a surface of an article usingmicro-contact printing.

Self-assembled monolayers (SAMs) are typically formed of moleculeshaving a functional group that selectively attaches (chemisorbs) to aparticular surface. The remaining part of the molecule interacts withneighboring molecules to form a relatively ordered monolayer.

A method disclosed in WO 96/29629 for applying a self-assembledmonolayer of a molecular species to a surface of an article comprisescoating a portion of a stamping surface of a stamp with a self-assembledmonolayer-forming molecular species, and transferring from the stampingsurface to a first portion of the article surface the molecular species,while applying to a second portion of the article surface contiguouswith the first portion a species that is not compatible with themolecular species. The stamp is maintained in contact with the articlesurface for a time sufficient to allow the self-assembledmonolayer-forming molecular species to spread evenly from the firstportion of the article surface to the second portion of the articlesurface. The spreading time is controlled in such a way as to providenon-coated gaps on the surface having a desirable dimension, such asfrom 100 nm to 10 μm. After removal of the stamp, an etchant is appliedto the surface. The etchant chosen is one that does not affect theself-assembled monolayer-forming molecular species. Thus, the etchantdissolves the surface material defined by said non-coated gaps on thearticle surface, and after removal of the self-assembled monolayer, apattern of non-etched material is provided on the article surface.

If the molecular species is lipophilic (i.e. hydrophobic), theincompatible Ispecies is hydrophilic. Furthermore, the incompatiblespecies selected is one that does not chemisorb to the article surface.

Typically, the molecular species is a hydrophobic liquid, such as amolecular species having a hydrophobic long-chain alkyl group, or iscarried in a hydrophobic liquid, and the incompatible species then is ahydrophilic liquid, such as water. Application of the incompatiblespecies is necessary according to WO 96/29629 to obtain the desirablesmooth and well-defined spreading of the molecular species over thearticle surface. Thus, if no incompatible species is present, theself-assembled monolayer-forming molecular species is said not tospontaneously spread and chemisorb between adjacent regions of stampingsurface.

A typical process used in micro-contact printing today is described inExample 2 in WO 96/29629. In this example, a gold coated siliconsubstrate is put into a petri dish half-filled with water and a stampincluding hexadecanethiol is brought into contact with the gold surface.The stamp and the substrate are either taken out of the water whilestill in contact and are then separated, or the stamp is separated fromthe gold coated substrate while still under water. The non-coated goldsurface is thereafter etched using a cyanide solution.

An important drawback of this method is that the stamp must be totallyimmersed in water because otherwise a monolayer forms at the surface ofthe water which leads to complete deposition on the entire articlesurface at the moment it is withdrawn from the water. To avoid such adeposition, the water may, as disclosed in Example 2 in WO 96/29629, bereplaced with several volumes of clean water while the article surfaceis still under water. However, this is a cumbersome procedure and thereis still a risk that residual SAM-forming species deposit on the articlesurface during withdrawal from the water.

As can be easily understood from the above, this micro-contact printingunder water is not an industrially suitable process. Thus, there is aneed to develop a more convenient process which may be used on anindustrial scale.

An object of the present invention is to alleviate the above problems,and to provide a micro-contact printing method which does not need to beperformed in a liquid incompatible with the self-assembledmonolayer-forming molecular species.

According to a first aspect of the invention, this and other objects areachieved with a method of applying a self-assembled monolayer of amolecular species to a surface of an article, comprising:

-   -   providing on at least a portion of a stamping surface of a stamp        a self-assembled monolayer-forming molecular species having a        first functional group selected to attach to said surface, and a        second functional group that is exposed when the species form a        monolayer, said second group being polar,    -   transferring the molecular species from the stamping surface to        a first portion of the article surface, and    -   allowing the molecular species to spread evenly from the first        portion of the article surface to a second portion of the        article surface, wherein the spreading is accomplished with the        stamp and the article is placed in a vacuum or in a gaseous        atmosphere, preferably in air.

According to a second aspect of the invention, this and other objectsare achieved with a method of applying self-assembled monolayers of twomolecular species to a surface of an article, comprising:

-   -   providing on at least a portion of a stamping surface of a stamp        a first self-assembled monolayer-forming molecular species        having a first functional group selected to attach to said        surface, and a second functional group that is exposed when the        species form a monolayer, said second group being polar,    -   transferring the molecular species from the stamping surface to        a first portion of the article surface,    -   providing on at least a portion of a stamping surface of a stamp        a second self-assembled monolayer-forming molecular species        having a first functional group selected to attach to said        surface, and a second functional group that is exposed when the        species form a monolayer, said second group being non-polar or        polar, preferably non-polar,    -   transferring the molecular species from the stamping surface to        said first portion of the article's surface coated with a        monolayer of said first molecular species,    -   allowing the second molecular species to spread evenly over the        first monolayer to a second portion of the article surface. The        spreading is preferably accomplished with the stamp and the        article is placed in a vacuum or in a gaseous atmosphere, more        preferably in air.

An advantage of the micro-contact printing methods according to theinvention is that the printing may be performed in a gaseous atmosphere,such as air. Thus, the stamp and the article do not need to be immersedin a liquid, such as water. Hence, the method according to the inventionis performed much more easily than any prior art method formicro-contact printing.

An additional advantage of the methods according to the invention isthat an improved controllability is provided. The amount of spreading iscontrolled by, for instance, temperature, contact time between stampsurface and article surface, choice of self-assembled monolayer-formingmolecular species, and concentration thereof.

Still another advantage of the method according to the second aspect ofthe invention is that a SAM having a lateral dimension ≦100 nm isobtainable.

Thus, according to a third aspect of the invention, an article having asurface comprising at least one isolated region of a self-assembledmonolayer of a molecular species is provided, wherein said region has alateral dimension within the range of from 1 to 100 nm.

An advantage of the article according to the invention is that it may beused to produce a device, such as a microelectronic device, comprisingan article surface having a very fine pattern of conducting,semi-conducting and/or insulating material(s). The applied monolayerherein can be a functional layer, but can alternatively be a layer ofphotoresist.

A particularly preferred example of such a very fine pattern is achannel between a source and a drain electrode in a field effecttransistor. The width of the channel, which is the smallest dimension inthe pattern, determines the switching speed of a transistor. With themethod of the invention, this width can be reduced, and thus thetransistor speed can be increased. The transistor can be ametal-oxide-semiconductor transistor on a semiconductor substrate, butis preferably a thin film transistor, that may be part of a displaydevice. In such a thin film transistor, various techniques can be usedto apply the layers, from a solution and by vapor deposition. It ispreferred that printing techniques are used therefore, particularly forlarge and flexible substrates.

Alternatively, very fine, nanometer-scaled patterns may be used fordefining nanoscaled structures.

In a preferred example, the pattern is provided on a substrate surface,the substrate comprising a stack of a first patterned layer ofelectrically conductive material defining a first and a second electrodeand a second layer of semiconductor material. The stack may comprise anyadhesion improving layer between the first and the second layer. Thelayer of semiconductor material is then patterned in accordance with adesired pattern with the method of the invention followed by an etchingstep wherein the monolayer acts as a photomask. In order to preventunderetching of the semiconductor material, it is preferably chosen tobe very thin, in the order of 5-10 nm. The desired pattern herepreferably comprises wire-shaped patterns, which extend from the firstelectrode to the second electrode. Combined with a gate dielectric and agate electrode, that can be provided on top of the semiconductormaterial, or as part of the substrate, a transistor is obtainedcomprising a nanowire semiconductor. As described in thenon-prepublished application EP02076428.8 (PHNL020286), the nanowire maycontain parts with a larger width, that can be used for memory or optoelectronic purposes.

Furthermore, according to a fourth aspect of the invention a method forproducing at least one nanowire, or a grid of nanowires, is provided.This method according to the invention comprises:

-   -   providing on a surface layer of a first material at least one        region of a self-assembled within the range of from 1 to 100 nm,        said surface layer being applied on a second layer of a second        material,    -   applying on the surface layer an etchant selected as one that        removes unprotected first material, but leaves the SAM and the        protected first material underlying said at least one region of        SAM unaffected,    -   applying an etchant selected as one that removes essentially the        entire second layer, and    -   isolating said first material, with or without said SAM, forming        at least one nanowire, or a grid of nanowires, having a lateral        dimension within the range of from 1 to 100 nm. The nanowire can        be made of a conducting, semi-conducting or isolating material.

At least one region of a self-assembled monolayer (SAM) of a molecularspecies is preferably provided on the surface layer of the firstmaterial by using the above disclosed method according to the secondaspect of the present invention.

Other features and advantages of the present invention will becomeapparent from the embodiments described hereinafter and the appendedclaims.

FIG. 1 schematically shows an embodiment of the method for applying aSAM according to the invention.

FIG. 2 schematically shows an embodiment of the method for applying twoSAMs according to the invention.

FIG. 3 shows a SEM graph of a ring transistor produced by applying a SAMaccording to an embodiment of the invention.

FIG. 4 shows a SEM graph of a ring transistor produced by applying a SAMaccording to an embodiment of the invention.

FIGS. 1 a-e schematically shows a first embodiment of a micro-contactprinting method according to the invention for applying a self-assembledmonolayer of a molecular species 1 to a surface 2 of an article 3.

The surface 2 of the article 3 preferably consists of a surface layer 2of a material other than the material constituting the article 3.

For instance, the article 3 might be a silicon substrate coated with asurface layer 2 of gold.

A stamp 4 having a surface 5 is used in said method. The surface 5preferably has a plurality of indentations 6 that form an indentationpattern, and define a plurality of protrusions 7, which outwardly facingsurfaces form a stamping surface 8.

Firstly, the stamping surface 8, typically the entire surface 5, isprovided with a self-assembled monolayer-forming molecular species 1having a polar functional group (see FIG. 1 a) that is exposed when thespecies form a monolayer.

The SAM-forming species 1 may be provided on the stamping surface 8 (orthe entire surface 5) by (a) directly coating the surface 8 with thespecies 1; (b) bringing the stamping surface 8 into contact with an “inkpad” comprising the species 1; (c) providing the species 1 in theinterior of the stamp and allowing the species 1 to diffuse through thestamp until it reaches the stamping surface 8, or (d) any otherapplication method known in the art, see, for instance, Libioulle, L;Bietsch, A; Schmid, H; Michel, B; Delamarche, E; Langmuir, 15(2), p300-304 (1999), and Blees et al, US 20020073861 A1.

The stamping surface 8 is thereafter brought into contact with a firstportion 9 of the article surface 2 and the molecular species 1 istransferred from the stamping surface 8 to said first portion 9 of thesurface 2 (see FIG. 1 b).

While stamping surface 8 and the first portion 9 of the article surface2 are still in contact, the molecular species 1 is allowed to spreadevenly from the first portion 9 to a second portion 10 of the articlesurface 2, see FIG. 1 c. This spreading is accomplished with the stamp 4and the article 3 placed in a gaseous atmosphere, preferably air. Thus,it is not necessary to apply a species not compatible with theself-assembled mono-layer forming molecular species 1, e.g. water, asdisclosed in WO 96/29629.

The stamp 4 and the article 3 may also be placed in a vacuum or in areduced pressure atmosphere.

SAM-forming molecular species 1 are usually of the general formulaR′-A-R″, wherein R′ is a functional group selected to attach to anarticle surface of a certain material, A is a spacer, and R″ is afunctional group that is exposed when the species form a SAM. Thus, R″defines the functionality of the SAM. For instance, if the exposedfunctional group R″ is hydrophilic, the SAM displays a hydrophilicexposed surface.

SAM-forming molecular species 1 may, however, also have the generalizedstructure R′-A-R″-A′-R′, wherein A′ is a second spacer or the same as A,or R′-A-R″-A′-R′″, wherein R′″ is the same or a different exposedfunctionality as R″. Additionally, species such as R′-A-R″-B andB-R′″-A′-R′-A-R′″-B′ may be chosen, wherein B and B′ are similar to A,do not prevent exposure of R′″ and R″ to the surrounding environment,and may be the same or different. It is to be understood that, in theabove general formulas, A and R″ or R′″ may not be distinguishable, butmay be continuous. For example, when A comprises an alkyl chain, and R″or R′″ comprises an alkyl functionality, A and R″ or R′″ together maysimply define an alkyl chain.

The article surface 2 can be made from a variety of electricallyconducting, insulating or semi-conducting materials.

The choice of the functional group R′, which is supposed to attach tothe article's surface 2, depends on the material of the article surface2.

A non-limiting exemplary list of suitable materials for the articlesurface 2 and preferred functional groups which chemisorb thereto aregiven below.

Sulphur-containing functional groups such as thiols, sulphides,disulphides, and the like firmly attach to metals, such as gold, silver,copper, cadmium, zinc, palladium, platinum, mercury, lead, iron,chromium, manganese, tungsten, and alloys thereof

Silanes and chlorosilanes firmly attach to doped or undoped silicon,quartz, glass, and oxide surfaces, such as chromium oxide, titaniumoxide, indium oxide, and tin oxide.

Carboxylic acids firmly attach to metal oxides, such as silica,aluminia, and other oxide surfaces, such as chromium oxide, titaniumoxide, indium oxide, and tin oxide, quartz, glass, and the like.

Nitriles and isonitriles firmly attach to platinum and palladium.

Hydroxarnic acids firmly attach to copper.

Other functional groups include acid chlorides, anhydrides, sulfonylgroups, phosphoryl groups, hydroxyl groups and amino acid groups.

Other materials for the article surface include germanium, gallium,arsenic, gallium arsenide, epoxy compounds, polysulfone compounds, andother polymeric materials.

The SAM-forming molecular species 1 used in the method according to thepresent invention could comprise any functional group selected to attachto a certain surface material. Hence, the method according to theinvention is suitable for any surface material as long as a SAM-formingspecies 1 may be attached thereto.

The important thing for the SAM-forming molecular species 1 used in thepresent method is that the exposed functional group (R″ and/or R′″) ispolar.

As used herein the term “polar functional group” means any functionalgroup having a more polar character than —CH₃. Such a polar group mayalso be referred to as hydrophilic or lipophobic.

A non-limiting exemplary list of suitable polar groups which may be usedin the method according to the invention is the following: —OH, —CONH,—CONHCO, —NCO, —NH₂, —NH—, —COOH, —COOR, —CSNH—, —NO₂, —SO₂ ⁻—, —COR,—COX, —ROR, —RCOR, —RCSR—, —RSR—, —PO₄ ²⁻, —OSO₃ ⁻—, —SO₃ ⁻—,NH_(x)R_(4-x), —COO⁻, —SOO⁻, —RSOR—, —CONR₂, —(OCH₂CH₂)_(n)OR (wheren=1−100), —PO₃H⁻, -2-imidazole, —N(CH₃)₂, —NR₂, —PO₃H₂, —CN, —SH, ahalogenated hydrocarbon, or any chemically possible combination of thesegroups.

In the above list, R is hydrogen or an organic group, such as ahydrocarbon or a halogenated hydrocarbon.

As used herein the term “hydrocarbon” includes alkyl, alkenyl, alkynyl,cycloalkyl, aryl, alkaryl, aralkyl, and the like. The hydrocarbon groupmay for example comprise methyl, propenyl, ethynyl, cyclohexyl, phenyl,tolyl, and benzyl groups. As used herein the term “halogenatedhydrocarbon” means halogenated derivates of the above describedhydrocarbons.

R may also be a biologically active species, such as an antigen, anantibody, or a protein, as known to persons skilled in the art. Thus, aSAM which selectively binds to various biological or other chemicalspecies can be provided. For instance, if R in the SAM-forming speciesis an antibody, the corresponding antigen may be selectively bound to asurface coated with the SAM-forming species.

X is a halogen atom, such as Cl, F, or Br.

Preferred polar groups which may be used in the method according to theinvention are the following:

-   -   —OH, —NCO, —NH₂, —COOH, —NO₂, —COH, —COCl, —PO₄ ²⁻, —OSO₃ ⁻,        —SO₃ ⁻, —CONH₂, —(OCH₂CH₂)_(n)OH, —(OCH₂CH₂)_(n)OCH₃ (where        n=1−100), —PO₃H⁻, —CN, —SH, —CH₂I, —CH₂Cl, and —CH₂Br.

The most studied combination of Surface material and SAM-forming species1 is a gold surface 2 and SAM-forming molecular species 1 comprising asulphur-containing group, such as a thiol group.

The SAM-forming molecular species 1 used in the method according to thepresent invention is preferably selected from the group consisting of:

-   -   an omega-functionalized thiol having the general formula        R′-A-R″, wherein R′ is —SH, A is —(CHR)_(n)— where R is H or        —CH₃, and n is an integer from 1 to 30, preferably from 12 to        30, more preferably from 16 to 20, and R″is a polar group,    -   a disulphide having the general formula R′″-A-S-S-A′-R″, wherein        R″ is a polar or a non-polar group, A and A′ independently are        —(CHR)_(n)— where R is H or —CH₃, and n is an integer from 1 to        30, preferably from 12 to 30, more preferably from 16 to 20, and        R″ is a polar group different from or the same as R′″, and    -   a thioether having the general formula R′″-A-S-A″-R″ or        R′″-A-S-A′-S-A″-R″, wherein R′″ is a polar or a non-polar group,        A, A′, and A″ independently are —(CHR)_(n)— where R is H or        —CH₃, and n is an integer from 1 to 30, preferably from 12 to        30, more preferably from 16 to 20, and R″ is a polar group        different from or the same as R′″.

The sulphur-containing groups, such as —SH, attach to the article'ssurface, and R″ is an exposed functional group of the SAM-formingmolecular species.

More preferably, the SAM-forming molecular species used in the methodaccording to the present invention is an omega-functionalized thiol.

Referring now to FIG. 1 c, the first portion 9 of the article's surface2 preferably comprises at least two isolated regions 9 a and 9 bseparated by the second portion 10. Thus, the molecular species 1 ispreferably transferred from the stamping surface 8 to the at least twoisolated regions 9 a and 9 b of the first portion 9, and then allowed tospread from each of the at least two isolated regions 9 a and 9 b of thefirst portion 9 toward each other. The stamping surface 8 and the firstportion 9 of the article's surface 2 preferably remain in contact for atime sufficient to provide a gap 11 having a predetermined dimensionbetween the spreading molecular species 1.

The width of the gap 11 is preferably within the range of from 50 nm to5 μm, more preferably from 100 nm to 2 μm.

The obtained width of the gap 11 depends on several factors affectingthe spreading process, which factors may be controlled.

Firstly, the time during which the stamping surface 8 is in contact withsaid first portion 9 of the article surface 2 affects the amount ofspreading.

Secondly, the concentration of the SAM-forming molecular species 1affects the amount of spreading. A higher concentration results infaster spreading.

Thirdly, the temperature at which said spreading is performed affectsthe amount of spreading. A higher temperature results in fasterspreading.

Fourthly, the type of SAM-forming molecular species 1 selected affectsthe amount of spreading.

Fifthly, the flux (diffusion rate) of the SAM-forming molecular species1 to the stamping surface 8 also affects the amount of spreading. Forinstance, if the SAM-forming species 1 is provided in the interior ofthe stamp 4, the flux will depend on the diffusion coefficient and theconcentration of species 1 in the stamp 4. The diffusion coefficient ofthe SAM-forming species 1 is affected by the size and shape of themolecular species 1 and by the interaction between the SAM-formingspecies 1 and the stamp material, generally rubber. Thus, the spreadingmay, to some extent, be controlled by selection of suitable stampmaterial(s) or any other modification of the stamp 4 as known in theart. For instance, a diffusion barrier, e.g. a thin film of a metal, apolymer, a ceramic, or a hybrid organic-inorganic material, may beincorporated in the stamp 4 to control the flux of the SAM-formingspecies 1. This diffusion barrier may be provided anywhere in the pathof diffusion of the SAM-forming species 1 within the stamp 4.

The size of the indentions 6 and protrusions 7 of the stamp 4 may alsohave some minor effect on the amount of spreading.

The relationship between surface tension (surface energy) (γ) andcontact angle (Θ) for a liquid (L) droplet on a solid (S) substratesurface in a gaseous (G) atmosphere, such as air, is expressed byYoung's law:γ_(SG)=γ_(SL)+γ_(LG) cos Θ  (I)

γ_(SG) denotes the surface tension between the substrate surface andair, γ_(SL) denotes the surface tension between the solid surface andthe droplet, and γ_(LG) denotes the surface tension between the dropletand air.

Spreading occurs when Θ≈0, thusγ_(SG)<γ_(SL)+γ_(LG)   (II)

During said spreading, unattached SAM-forming species diffuse over themonolayer. The diffusion of these non-attached molecular species on themonolayer is, on a molecular scale, quite similar to the behavior of aliquid droplet on a substrate surface. Thus, Young's law is, at leastapproximately, applicable to describe the spreading process.

The surface tension between the monolayer and air corresponds to γ_(SG)in Young's law.

The surface tension between the diffusing unattached SAM-forming speciesand the monolayer corresponds to γ_(SL) in Young's law.

The surface tension between the diffusing unattached SAM-forming speciesand air corresponds to γ_(LG) in Young's law.

For a gold surface in air, γ_(SG)>500 mJ/m².

A monolayer of the omega-functionalized thiol comprising a non-polarmethyl group, HS—(CH₂)₁₇—CH₃, in air gives γ_(SG) of about 20 mJ/m².

A monolayer of the omega-functionalized thiol comprising a polarcarboxylic acid group, HS—(CH₂)₁₅—COOH, in air gives γ_(SG) of about 50mJ/m².

Thus, the above thiol comprising a non-polar exposed functional groupwill not spread on its own monolayer, i.e. the thiol is described asautophobic, because γ_(SG) is relatively low.

However, the above thiol comprising a polar exposed functional groupwill spread on its own monolayer because γ_(SG) is relatively high.

Referring now to FIG. 1 d, as the desirable gap width has been obtained,the stamp 4 is removed from the article surface 2, and an article 3having a surface 2 comprising at least one area 12, preferably aplurality of areas 12, coated with SAMs 1 is obtained, said areas 12being separated by a small gap 11.

After removal of the stamp 4, an etchant is applied to the article'ssurface 2. The etchant selected does not affect the SAM-formingmolecular species 1, but etches the material used for the articlesurface 2, e.g. gold. Thus, the surface material defined by the gap 11on the article surface 2 is removed by the etchant and the areas 12coated with SAM are left undisturbed.

After the etching process, the SAM 1 is either removed, resulting in apatterned article surface 2 having protruding areas 2′ of surfacematerial separated by an etched area 11′ corresponding in size to saidgap 11 (see FIG. 1 e), or the SAM 1 is kept on the article surface 2′,for instance, to act as an adhesion promoter during application of anadditional layer thereon or because it may actually have a favorableeffect on the function of the resulting device comprising the article.

Delamarche et al, J Am Chem Soc, 124, p 3835 (2002) describes a methodof producing an article surface having an inversed pattern in relationto the above. The method comprises the application of a secondSAM-forming species to non-coated areas of an article surface that ispartially coated, by micro-contact printing, with a first SAM-formingspecies. The SAM-forming species are selected in such a way that theonly the first SAM is affected by a certain etchant, and the second SAMis not.

The above disclosed method according to the invention may be used toprovide such an inversed pattern.

Alternatively, instead of etching, selective deposition using e.g.electroless deposition, electrodeposition, particle/polymer adsorptionfrom solution, surface-initiated polymerization, or chemical vapordeposition, may be performed using the partially SAM-coated articlesurface obtained by the method according to the present invention.

FIGS. 2 a-e schematically shows parts of a second embodiment of amicro-contact printing method according to the invention. All of thesteps disclosed above, and shown in FIGS. 1 a-d, up to the removal ofthe stamp 4 are performed. In FIG. 2 a, no spreading of SAM-formingspecies 1 is shown to have occurred. However, it might, for reasonsindicated below, be advantageous to allow some spreading to occur.

After removal of the stamp 4, the stamp 4 is cleaned by removing anyresidual SAM-forming species 1 and a second SAM-forming molecularspecies 13 having preferably a non-polar functional group is provided,by any of the methods disclosed above, on the stamping surface 8, seeFIG. 2 a.

Instead of cleaning the stamp 4, a second stamp having either a stampingsurface identical to the one used for transfer of the first molecularspecies 1, or a stamping surface with a different pattern and/ordimensions than the first stamp may be used.

Subsequently the stamping surface 8 is again brought into contact withthe first portion 9 of the article surface 2 coated with the firstSAM-forming species 1, see FIG. 2 b. If an identical stamping surface 8is used for transfer of both SAM-forming species 1 and 13, it might, dueto reasons of alignment, be advantageous to allow some spreading ofmolecular species 1 to occur before application of the secondSAM-forming species 13.

The molecular species 13 cannot chemisorb to the first portion 9 of thearticle surface 2 since the first SAM-forming species 1 is alreadyattached thereto. However, the second SAM-forming species 13 willdiffuse over the first SAM 1 until it reaches the second uncoatedportion 10 of the article surface 2, see FIG. 2 c. As soon as a fewmolecules of this second SAM-forming species 13 attach to the articlesurface 2 and form a second SAM 13, the diffusion will stop because theSAM-forming species 13 having a non-polar exposed functional group isautophobic, i.e. the molecules will not spread on their own monolayer.Thus, this strips of the second SAM-forming species 13 having very smalllateral dimensions, such as within the range of from 1 to 40 nm, areprovided.

Thus, an article 3 having a surface 2 comprising at least one isolatedregion of a self-assembled monolayer of a molecular species 13 isprovided, wherein said region having a lateral dimension within therange of from 1 to 40 nm.

The first molecular species 1 is then again applied and attached to theremaining uncoated article surface 2, see FIG. 2 d. The first molecularspecies 1 may be applied by dip coating, vapor deposition, spraying, orby transfer using a flat stamp without indentations or protrusions.

An etchant, selected as one that removes the first SAM-forming species 1and etches the underlying material of the article surface 2, but thatdoes not disturb the second SAM-forming species 13, is applied to thearticle surface 2. After the etching process the second SAM 13 is, if sodesired, removed, resulting in a patterned article surface 2 havingprotruding areas 2′ of surface material separated by etched areas 14,see FIG. 2 e.

An example of the first SAM-forming molecular species 1 ispentaerythritol-tetrakis (3-mercaptopropion-ate).

An example of the second SAM-forming molecular species 13 is1-octadecylthiol.

In a third embodiment of the present invention, the above secondSAM-forming molecular species 13 comprises a polar second functionalgroup. Thus, such a molecular species is not autophobic, which meansthat the molecules will spread on their own monolayer and provideSAM-strips with a larger lateral dimension, such as within the range offrom 40 to 100 nm, or even larger, than in the second embodimentdisclosed above.

Thus, an article 3 having a surface 2 comprising at least one isolatedregion of a self-assembled monolayer of a molecular species 13 isprovided, said region having a lateral dimension within the range offrom 40 to 100 nm.

The present invention also relates to a method for producing at leastone nanowire, or a grid of nanowires, of conducting, semi-conducting orinsulating material. The above disclosed article having a surface 2comprising at least one isolated region of a self-assembled monolayer ofa molecular species 13, said region having a lateral dimension withinthe range of from 1 to 100 nm, is preferably utilized in this method

Thus, in an embodiment of this method according to the invention, anarticle comprising a surface layer of a first material and at least onesecond layer of a second material located underneath the surface layeris utilized in the above disclosed method according to the second aspectof the invention. After the above disclosed removal of the firstSAM-forming species 1 and the underlying surface layer material (firstmaterial), a second etchant, selected as one that removes the secondmaterial constituting the entire second layer, including the areaslocated under the second SAM 13, is applied. As the second layer isremoved by the second etchant at least one isolated nanowire, or anisolated grid of nanowires, of non-etched surface material, e.g. gold,possibly still coated with the second SAM-forming species 13, isseparated from the article. The second SAM-forming species 13, if stillpresent, is subsequently removed from the nanowire or the grid or iskept if so desired. Thus, at least one nanowire, or a grid of nanowires,having a lateral dimension within the range of from 1 to 100 nm isprovided.

As used herein, the term “nanowire” is not restricted to wires having asymmetric cross-section. It might just as well be a wire having, forinstance, an essentially rectangular cross-section. Such a wire mightalso be referred to as a “nanoribbon”.

Examples of devices comprising such nanowires, or a grid of nanowires,are field emitters, wire grid polarizers, and microelectronic devices.

The micro-contact printing methods according to the invention may beperformed using any known printing principle, such as standard printing,roll printing or variants thereof, as disclosed in the introduction.

The method according to the present invention is useful for theproduction of, for example, electronic devices, such as transistors,biosensors, liquid crystalline displays, optical devices, or any otherarticles comprising a surface (curved or non-curved) having amicro-structured pattern.

The invention will now be further elucidated by means of the followingnon-limiting examples, which show that the distance of diffusion of aSAM-forming species, applied according to the method of the presentinvention, increases with contact time between the stamping surface andthe article surface.

EXAMPLE 1

An omega-functionalized thiol comprising a polar carboxylic acid group,HS—(CH₂)₁₃—COOH, was dissolved in ethanol, yielding a thiolconcentration of 25 mM.

Other organic solvents, such as methanol, 2-butanone, acetone,1-propanol, 2-propanol toluene, o-xylene, p-xylene, tetrahydrofyran, ordimethylformamide, may also be used. However, ethanol is the preferredsolvent.

A stamp having a stamping surface defined by the outward-facing surfacesof several protrusions was provided with the dissolved thiol.

The distance between the protrusions studied in this example was 2.5 μmand the height of the protrusions perpendicular to the stamping surfacewas 2.1 μm. The protrusions corresponded in size to the source and drainelectrodes of a transistor structure.

A layer of titanium (Ti) having a thickness of 5 nm and, on top thereof,a layer of gold (Au) having a thickness of 20 nm were sequentiallyapplied using thermal evaporation on a silicon substrate coated with athermal oxide having a thickness of 200 nm. The titanium layer here actsas an adhesive between the gold and the oxide. Other substances, such aschromium (Cr), molybdenum (Mo), titanium-tungsten (TiW), may also beused as an adhesion layer.

Said thiol was transferred from the stamping surface to a first portionof the gold-coated silicon substrate using the method disclosed aboveand shown in FIGS. 1 a-e, thereby forming a self-assembled monolayer onthe gold surface. The first portion of the gold surface consists ofseveral isolated regions separated by a second portion of the goldsurface.

The stamping surface and the gold surface were in contact for 60 sbefore removal of the stamp. During this period of time, the thiolspread from each isolated region towards the adjacent region, creating agap width, herein also referred to as the source-drain distance, ofabout 0.85 μm between the spread thiols. Thus, the thiols diffused about0.8 μm during the contact period of 60 s.

The temperature during the contact period was 23° C.

The resulting substrate having a partially SAM-coated gold surface wasthen immersed in an aqueous solution containing 1.0 M KOH, 0.1 M K₂S₂O₃,0.01 M K₃Fe(CN)₆, and 0.001 M K₄Fe(CN)₆ for 8 minutes at 23° C. Thisetchant removes the uncoated gold surface defined by the gap, but doesnot affect the thiol, thus leaving the areas coated with the thiolnon-etched.

The titanium layer was revealed in the area where gold was removed. Thetitanium in this area was subsequently removed by immersing thesubstrate in an aqueous solution containing 1.5 M H₂O₂ and 1.0 M(NH₄)₂HPO₄ at 40° C.

After these etching processes, the thiol was removed by placing thesubstrate in a microwave plasma reactor in an argon atmosphere at apressure of 0.25 mbar for 1 minute, thus providing a patterned goldsurface having protruding areas of gold separated by etched areas with alateral dimension of about 850 nm.

A SEM graph of a ring (shaped) transistor produced according to thisExample is shown in FIG. 3. The outer ring therein defines the drainelectrode and the inner ring defines the source electrode. The channelis present between the source and the drain electrode. Semiconductormaterial, gate dielectric and gate electrode are not shown, but can beapplied in known manner. The semiconductor material is for instanceamorphous silicon or an organic semiconductor or it is provided asnanowires of semiconductor material.

It shall be noted that the switching frequency of a transistor decreasesquadratically with the source-drain distance.

EXAMPLE 2

Example 1 was repeated with the exceptions that the contact time was 160s, and that the distance between the protrusions studied in this Examplewas 5.0 μm.

The gap provided had a width (source-drain distance) of about 2.4 μm.Thus, the thiols diffused about 1.3 μm during the contact period of 160s.

A SEM graph of a ring transistor produced according to this Example isshown in FIG. 4.

While the invention has been described in detail and with reference tospecific embodiments thereof, it will be apparent to persons skilled inthe art that various changes and modifications can be made thereinwithout departing from the spirit and scope thereof.

1. A method of applying a self-assembled monolayer of a molecularspecies to a surface of an article, comprising: providing on at least aportion of a stamping surface of a stamp a self-assembledmonolayer-forming molecular species having a first functional groupselected to attach to said surface, and a second functional group thatis exposed when the species form a monolayer, said second group beingpolar, transferring the molecular species from the stamping surface to afirst portion of the article surface, and allowing the molecular speciesto spread evenly from the first portion of the article surface to asecond portion of the article surface, characterized in that thespreading is accomplished with the stamp and the article is placed in avacuum or in a gaseous atmosphere.
 2. A method of applyingself-assembled monolayers of two molecular species to a surface of anarticle, comprising: providing on at least a portion of a stampingsurface of a stamp a first self-assembled monolayer-forming molecularspecies having a first functional group selected to attach to saidsurface, and a second functional group that is exposed when the speciesform a monolayer, said second group being polar, transferring themolecular species from the stamping surface to a first portion of thearticle surface, characterized by providing on at least a portion of astamping surface of a stamp a second self-assembled monolayer-formingmolecular species having a first functional group selected to attach tosaid surface, and a second functional group that is exposed when thespecies form a monolayer, said second group being polar or non-polar,transferring the molecular species from the stamping surface to saidfirst portion of the article surface coated with a monolayer of saidfirst molecular species, and allowing the second molecular species tospread evenly over the first monolayer to a second portion of thearticle's surface.
 3. A method according to claim 2, wherein thespreading is accomplished with the stamp and the article is placed in avacuum or in a gaseous atmosphere.
 4. A method according to claim 3,wherein the second functional group of the second self-assembledmonolayer-forming molecular species is non-polar.
 5. A method accordingto claim 1, wherein the gaseous atmosphere is air.
 6. A method accordingto claim 1, wherein the article′ surface is a metal surface and theself-assembled monolayer-forming molecular species is selected from thegroup consisting of: an omega-functionalized thiol having the generalformula R′-A-R″, wherein R′ is —SH, A is —(CHR)_(n)— where R is H or—CH₃, and n is an integer from 1 to 30, and R″ is a polar group, adisulphide having the general formula R′″-A-S-S-A′-R″, wherein R′″ is apolar or a non-polar group, A and A′ independently are —(CHR)_(n)— whereR is H or —CH₃, and n is an integer from 1 to 30, and R″ is a polargroup, different from or the same as R′″, and a thioether having thegeneral formula R′″-A-S-A″-R″ or R′″-A-S-A′-S-A″-R″, wherein R′″ is apolar or a non-polar group, A, A′, and A″ independently are —(CHR)_(n)—where R is H or —CH₃, and n is an integer from 1 to 30, and R″ is apolar group, being different from or the same as R′″.
 7. A methodaccording to claim 6, wherein the polar group R″ is a functional groupselected from the group consisting of —OH, —NCO, —NH₂, —COOH, —NO₂,—COH, —COCl, —PO₄ ²⁻, —OSO₃ ⁻, —SO₃ ⁻, —CONH₂, —(OCH₂CH₂)_(n)OH,—(OCH₂CH₂)_(n)OCH₃, —PO₃H⁻, —CN, —SH, —CH₂I, —CH₂Cl, and —CH₂Br, whereinn is an integer from 1 to
 100. 8. An article having a surface comprisingat least one isolated region of a self-assembled monolayer of amolecular species, characterized in that said region has a lateraldimension within the range of from 1 to 100 nm.
 9. A method of producingat least one nanowire, or a grid of nanowires, characterized in that themethod comprises: providing a surface with a second layer of a secondmaterial and providing a surface layer of a first material thereon,providing on the surface layer at least one region of a self-assembledmonolayer (SAM) of a molecular species, said region having a lateraldimension within the range of from 1 to 100 nm, applying on the surfacelayer an etchant selected as one that removes unprotected firstmaterial, but leaves the SAM and the protected first material underlyingsaid at least one region of SAM unaffected, applying an etchant selectedas one that removes essentially the entire second layer, and isolatingsaid first material, with or without said SAM, thus forming at least onenanowire or a grid of nano-wires.
 10. A method of manufacturing anelectronic device comprising the step of providing a patterned layerwith a desired pattern on a surface of an article, characterized in thatthe patterned layer is defined by providing a monolayer according toclaim
 1. 11. A method as claimed in claim 10, characterized in that anelectronic device is provided with a field effect transistor having asource and a drain electrode, a channel, a gate electrode and a gatedielectric, and that the desired pattern defines the channel between thesource and the drain electrode.
 12. A method as claimed in claim 10,characterized in that the article comprises at its surface a stack of afirst patterned layer of electrically conductive material and a secondlayer of semiconductor material, in which first layer a first and asecond, mutually isolated electrode are defined; the desired pattern issuch that a perpendicular projection thereof on the first layer overlapswith the first and the second electrode; after defining the pattern, thesecond layer is etched with an etchant selected as one that removesunprotected semiconductor material, but leaves the pattern and theprotected semiconductor material underlying the pattern unaffected. 13.A method of manufacturing an electronic device comprising the step ofproviding nanowires on a substrate, characterized in that the nanowiresobtainable with the method according to claim 9 are provided.